jenkins: Fix Windows build to specify Studio version
jenkins: Fix Windows build to specify Studio version

If there are multiple versions available, cmake won't choose the Visual
Studio version selected by vsvars. So we have to explicitly specify the
generator to use.

This allows grpc-java to run on the shared Windows workers instead of
its own specialized instance.

carl-mastrangelo/grpc-java,stanley-cheung/grpc-java,pieterjanpintens/grpc-java,zhangkun83/grpc-java,nmittler/grpc-java,simonhorlick/grpc-java,rmichela/grpc-java,pieterjanpintens/grpc-java,anuraaga/grpc-java,ejona86/grpc-java,elandau/grpc-java,dapengzhang0/grpc-java,carl-mastrangelo/grpc-java,rmichela/grpc-java,zhangkun83/grpc-java,stanley-cheung/grpc-java,simonhorlick/grpc-java,elandau/grpc-java,ejona86/grpc-java,zhangkun83/grpc-java,zhangkun83/grpc-java,elandau/grpc-java,grpc/grpc-java,pieterjanpintens/grpc-java,grpc/grpc-java,pieterjanpintens/grpc-java,dapengzhang0/grpc-java,carl-mastrangelo/grpc-java,dapengzhang0/grpc-java,grpc/grpc-java,simonhorlick/grpc-java,nmittler/grpc-java,rmichela/grpc-java,zpencer/grpc-java,zpencer/grpc-java,nmittler/grpc-java,zpencer/grpc-java,carl-mastrangelo/grpc-java,stanley-cheung/grpc-java,zpencer/grpc-java,anuraaga/grpc-java,stanley-cheung/grpc-java,rmichela/grpc-java,elandau/grpc-java,anuraaga/grpc-java,dapengzhang0/grpc-java,ejona86/grpc-java,ejona86/grpc-java,simonhorlick/grpc-java,grpc/grpc-java